Re: [PATCH 4/6] vhost: reset invalidate_count in vhost_set_vring_num_addr()

From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 4/6] vhost: reset invalidate_count in vhost_set_vring_num_addr()
Date: Tue, 23 Jul 2019 21:25:22 +0800	[thread overview]
Message-ID: <4e52f1cb-f805-18f6-d50b-1379298de2e3__41826.9135219575$1563888333$gmane$org@redhat.com> (raw)
In-Reply-To: <20190723042143-mutt-send-email-mst@kernel.org>

On 2019/7/23 下午5:17, Michael S. Tsirkin wrote:
> On Tue, Jul 23, 2019 at 03:57:16AM -0400, Jason Wang wrote:
>> The vhost_set_vring_num_addr() could be called in the middle of
>> invalidate_range_start() and invalidate_range_end(). If we don't reset
>> invalidate_count after the un-registering of MMU notifier, the
>> invalidate_cont will run out of sync (e.g never reach zero). This will
>> in fact disable the fast accessor path. Fixing by reset the count to
>> zero.
>>
>> Reported-by: Michael S. Tsirkin <mst@redhat.com>
>> Fixes: 7f466032dc9e ("vhost: access vq metadata through kernel virtual address")
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>> ---
>>   drivers/vhost/vhost.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 03666b702498..89c9f08b5146 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -2074,6 +2074,10 @@ static long vhost_vring_set_num_addr(struct vhost_dev *d,
>>   		d->has_notifier = false;
>>   	}
>>   
>> +	/* reset invalidate_count in case we are in the middle of
>> +	 * invalidate_start() and invalidate_end().
>> +	 */
>> +	vq->invalidate_count = 0;
> I think that the code is ok but the comments are not very clear:
> - we are never in the middle since we just removed the notifier

If I read the code correctly, mmu_notifier_unregister() can only 
guarantee to wait for the pending method to complete. So we can have:

invalidate_start()

mmu_notifier_unregister()

invalidate_end()

> - the result is not just disabling optimization:
>    if notifier becomes negative, then later we
>    can think it's ok to map when it isn't since
>    notifier is active.

I don't get how it could be negative, the only possible thing is to have 
a positive value.

Thanks

>
>>   	vhost_uninit_vq_maps(vq);
>>   #endif
>>   
>> -- 
>> 2.18.1
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